The applicant has previously proposed, for instance in Japanese patent laid-open publication No. 8-21492 which corresponds to the U.S. patent application Ser. No. 08/497,557 filed Jun. 30, 1995, a torque splitting device which, provided in parallel with a conventional differential device, controls the simulated rolling resistance to each of the right and left or front and rear axles and boosts the rotational speed of the axle encountering a lower rolling resistance. Thereby, the torque distribution ratio to the right and left axles can be positively changed depending on the steering wheel steering angle and the vehicle speed to the end of improving the steering performance of the vehicle. The contents of the above mentioned United States patent application are hereby incorporated in this application by reference.
As illustrated in FIG. 10, this previously proposed torque splitting device T comprises an oil pressure pump 32 producing an output pressure that depends on the vehicle speed, a regulator Re for adjusting the output pressure to a prescribed level, a pair of wet hydraulic multi-disk clutches Ca and Cd for producing simulated rolling resistances, a pressure regulating valve 30 consisting of a linear solenoid valve for determining a torque distribution ratio for the right and left (or front and rear) wheels according to the turning radius or the road resistance, and controlling the engagement forces of the clutches Ca and Cd so as to achieve a desired torque distribution ratio by adjusting the oil pressure for each of the clutches to a target value, an electronic control unit 29 for computing the target oil pressures, and controlling the electric current for the pressure regulating valve 30, and a planetary gear mechanism P which is connected to the wet hydraulic multi-disk clutches Ca and Cd and actually distributes the torque. The output of the engine E forwarded to the torque splitting device T via the transmission TM can be thus appropriately distributed to the right and left (or front and rear) axles 5L and 5R depending on the operating condition of the vehicle.
The operating response of such a clutch is affected by the viscosity of the actuating oil, and tends to drop under a low temperature condition because of an increase in viscosity. Because the clutch is typically disengaged by removal of the actuating oil from the clutch cylinder, the response delay is particularly significant when disengaging the clutch under a low temperature condition. The reduction in the response of the clutch due to an increase in the flow resistance to the actuating oil means that the difference in rotational speed between the right and left wheels may remain even after the steering wheel is brought back to the neutral position. This is not desirable because it causes discomfort to the vehicle operator.
Also, because the output of the oil pressure pump depends on the vehicle speed, the oil pressure pump may not be able to produce a sufficient oil pressure to appropriately operate the wet hydraulic multi-disk clutch in a low speed range. The volumetric efficiency of the oil pressure pump, which typically consists of a gear pump or a cam pump using a trochoidal or other piston element, is known to be affected by the viscosity or the temperature of the oil. When the oil temperature is high, and the viscosity of the oil is therefore low, the volumetric efficiency of the oil tends to drop. Therefore, for instance when a certain oil pressure target value is supplied to the pressure control valve so as to create a certain difference in the driving force between the right and left wheels to accommodate a turning maneuver, the oil pressure pump may not be able to produce the required oil pressure if the rotational speed of the engine is low and/or the oil temperature is high. Under such a condition, when the rotational speed of the oil pressure pump is accelerated from a low speed range involving an insufficient output pressure of the pump, it is possible for the wet hydraulic multi-disk clutch to abruptly engage as soon as the output pressure of the pump reaches a prescribed value. This means a discontinuity in the torque distribution control, and is not desirable again as it causes a discomfort to the vehicle operator.